Use a generated texture for Grease Pencil
Grease Pencil Materials don't support shader nodes (yet). Link to the docs .
Since you can't use nodes, you can't use procedural textures like Bricks or Noise Textures.
You can still use a texture for the stroke or the fill in the material properties.
However, the controls are limited to Blend between a plain color and the texture, and Mapping (offset, rotation and scale).
Using a raster seamless texture you can achieve this kind of result :